1. Field of the Invention
The present invention relates to an electroluminescence display device, and more particularly, to an electroluminescence display device that may prevent brightness non-uniformity due to a voltage drop in a display region.
2. Discussion of the Background
A variety of flat panel display devices have been recently developed to replace the conventional cathode ray tube (CRT). Generally, flat panel display devices are either emissive or non-emissive types. The emissive display devices, which do not require a separate light source, include flat CRTs, plasma display panel devices, vacuum fluorescent display devices, field emission display devices, and inorganic/organic electroluminescent display devices. The non-emissive display device includes liquid crystal display devices. The organic electroluminescent display device has drawn much attention because it is emissive, it does not consume a lot of power, and it is highly efficient. Further, it may be thin and lightweight, and it may have a wide viewing angle and quick response time.
The organic electroluminescent display device emits light having a specific wavelength by energy generated from exitons, which are formed when electrons and holes, injected through an anode and a cathode, recombine in an organic thin film.
A stacked organic electroluminescent unit of the organic electro-luminescent display device includes a first electrode (an anode), an intermediate layer including at least an emitting layer (EML), and a second electrode (a cathode). To increase the device's light emission efficiency, electrons and holes may be transferred to the organic EML. Accordingly, an electron transport layer (ETL) can be disposed between the cathode and the organic EML, and a hole transport layer (HTL) can be disposed between the anode and the organic EML. Also, a hole injection layer (HIL) can be disposed between the anode and the HTL, and an electron injection layer (EIL) can be disposed between the cathode and the ETL.
The organic electroluminescent display device may be a passive matrix (PM) or active matrix (AM) type depending upon its driving method. In a PM device, anodes and cathodes are simply arranged in columns and rows, respectively, and a row driving circuit sequentially supplies scanning signals to one cathode (i.e. row) at a time. Also, a column driving circuit supplies data signals to each pixel. On the other hand, the AM device controls signals inputted to each pixel using a thin film transistor (TFT). Hence, an AM device is typically used to display moving images since it can process a large number of signals.
FIG. 1A is a plan view of a conventional organic electroluminescent display device, and FIG. 1B is a cross-sectional view taken along line I-I of FIG. 1A.
The AM organic electroluminescent display device of FIG. 1A has a display region 20 that includes an organic light emitting diode (OLED) on a transparent substrate 11, and the display region 20 is sealed by a sealing member (not shown), such as a metal cap, and a sealing unit 80. The display region 20 comprises a plurality of pixels, and each pixel may include a TFT. A plurality of driving lines VDD 31 is disposed in the display region 20. The driving lines 31 supply power to the display region 20, and they are coupled to a terminal region 70 through a driving power supply line 32 disposed outside the display region 20.
As FIG. 1B shows, a TFT layer 10a for applying electrical signals to an electroluminescent unit in the display region 20 is formed on a surface of a substrate 11, and a pixel layer 10c that includes an electroluminescent unit is formed on the TFT layer 10a. An insulating layer 10b is interposed between the TFT layer 10a and the pixel layer 10c. 
A via hole formed in the insulating layer 10b couples the TFT layer 10a with the pixel layer 10c. FIG. 1C is a magnified cross-sectional view showing portion “A” of FIG. 1B. Referring to FIG. 1C, a first insulating layer 18a may be formed on the drain electrode 17b, a second insulating layer 18b may be formed on the first insulating layer 18a, and via holes 18′a and 18′b may be formed in the insulating layers 18a and 18b. 
With a front emission type electroluminescence display device, a reflection layer 19b may be formed below a first electrode layer 19a. The first electrode layer 19a supplies electrical signals to an intermediate layer 19c and is coupled to the drain electrode 17b of the TFT layer 10a through the via holes 18′a and 18′b. 
Further, if the first electrode layer 19a is an anode, conventionally, the first electrode layer 19a may be formed of a transparent conductive oxide, such as, for example, indium tin oxide (ITO) having a large work function, and the reflection layer 19b may be formed of Al or Al/Nd. However, the transmittance of electrical signals from the drain electrode 17b to the electroluminescent unit disposed in an opened region defined by a pixel defining layer 19d may be adversely affected due to reduced conductivity resulting from an interface oxide layer formed between an ITO first electrode layer 19a and an Al/Nd metal reflection layer 19b. This can reduce brightness or cause brightness non-uniformity in the display region, thereby reducing image quality.